To date, six human efficacy trials of candidate HIV vaccines have been completed. These trials have evaluated four different vaccine strategies have been completed. In the RV144 Thai trial a canarypox prime followed by boosts with canarypox and gp120 envelope protein were evaluated. Rv144 is the only trial to demonstrate that a vaccine candidate can protect against HIV acquisition, although protection was modest. The other trials failed to demonstrate any protection against HIV acquisition. Further, in the STEP and Phambili trials more people receiving the vaccine than the placebo became infected, demonstrating that the HIV vaccines have the potential to enhance susceptibility to HIV infection. Similarly, rhesus macaques chronically infected with Ad5 and then immunized with a replication defective Ad5 SIVmac239 Gag-Pol-Nef vaccine were more susceptible to penile SIV infection than unimmunized rhesus macaques. We will use this animal model to understand how an HIV vaccine enhanced HIV acquisition. In particular we will determine 1) if Ad5 persistence in tissues of the foreskin leads to increased T cell recruitment to, and retention in, that is amplifid by 3 Ad5 vector immunizations; 2) if Ad5 infected RM have increased numbers of activated CD4+ T cells/TH17 cells in the foreskin after immunization compared to Ad5- RM; 3) ifAd5 infected RM have more Ad5 specific CD4+ T cells and SIV-specific CD4+ T cells in the foreskin after immunization compared to Ad5- RM; 4) if Ad5 infected RM have more innate immune signals emanating from epithelial cells, DC and others in the foreskin that drive T cell recruitment and survival after immunization compared to Ad5- RM. Understanding the underlying mechanisms behind the vaccine-enhanced HIV susceptibility in the STEP trail will allow the field to avoid testing harmful HIV vaccines in people.